This invention relates to a heat treating apparatus and method of operating the same for dehydrating and sintering, for example, an optical fiber preform (porous material) to fabricate a transparent optical fiber preform.
Heretofore, a heat treating apparatus for heat treating a material to be treated such as an optical fiber preform has, as shown in FIG. 4, a core tube 2 for containing and heating a porous optical fiber preform 1 of a material to be heated, and a furnace body 4 surrounding the outer periphery of the core tube 2, a heater 5 disposed in the body 4 for heating the preform 1 through the core tube 2. In this case, the preform 1 supported by a supporting rod 6 and disposed at the center of the core tube 2 is heated in a suitable gas atmosphere in the tube 2, dehydrated, and sintered to form a transparent preform. Predetermined gas is supplied by core tube gas supply means 8 having a feed conduit 7 to the core tube 2, and the reaction gas with the preform 1, moisture and excessive gas are intaken and exhausted from exhaust means 10 having an exhaust conduit 9 through an exhaust gas treating unit 11 via a blower 12. A throttle valve for regulating the strength of the exhaust gas from the core tube 2 and a pressure gauge 14 for monitoring the internal pressure in the tube 2 are ordinarily provided in the exhaust conduit 9 of the exhaust means 10.
Most of the core tube 2 is formed of a quartz to endure against high temperatures and to avoid detrimental impurities in the optical fiber. Carbon is used as a material for the heater 4 and as a heat insulating material of the interior of the body 4 to obtain sufficiently high temperatures in a heating furnace 3.
When the carbon is used at high temperatures, the carbon is oxidized in the air and loses its shape. Thus, the carbon must be used in nonoxygen atmosphere. This method requires inert gas inside the body 4. In this case, in order to prevent air from entering through a gap between the body 4 and the core tube 2 and prevent oxygen from diffusing, the pressure in the body 4 must be maintained at a higher level than the external pressure of the body 4.
However, since the core tube 2 is made of a quartz and is heated at high temperatures at the center of the body 4, the tube 2 is softened to be deformable, and might expand or contract due to the pressure relationship between the inside and the outside of the tube 2. The higher the temperature of heater 5, the easier tube deformation becomes at lower pressure differences.
More specifically, the core tube 2 expands in case of P.sub.1 -P.sub.2 .gtoreq.K.sub.1, and the core tube 2 contracts in case of P.sub.1 -P.sub.2 .gtoreq.K.sub.2, where P.sub.1 is the pressure inside the tube 2, and P.sub.2 is the pressure outside the tube 2, and K.sub.1, K.sub.2 are values determined according to the size and temperature of the core tube 2.
From the abovementioned fact, the apparatus must be operated by holding the pressure in the tube 2 in a range for not deforming the tube 2. Further, the pressure P.sub.1 must not be higher nor lower than the lowest pressure P.sub.2 necessary to prevent the carbon from oxidizing.
However, the pressure varies according to the alteration for setting the flow rate of atmospheric gas, or the temperature of the heater and the pressure in the tube 2 varies according to the variation in the pressure in the exhaust system. There thus arises a disadvantage that the variation in the pressure causes the tube 2 to collapse or be damaged and to disable the heat treatment.